The polyphenol compound 2,2-bis (4-hydroxyphenyl) propane, also called para, para-diphenylolpropane or bisphenol-A (“BPA”), is generally prepared by reacting phenol and acetone in the presence of an acidic condensation catalyst along with a catalyst promoter or cocatalyst to increase the reaction rate and selectivity of the condensation catalyst. U.S. Pat. No. 2,468,982, incorporated herein by reference, disclosed the use of mercapto-substituted aliphatic carboxylic acids as catalyst promoters to increase the condensation reaction rate between phenols and ketones. It was later disclosed in U.S. Pat. No. 2,730,552, incorporated herein by reference, that contact time in the acid-catalyzed reaction between phenol and ketones was improved by the use of methyl mercaptans as promoter. Not only was the contact time reduced, but the use of gaseous methyl mercaptan in the reaction zone permitted running the reaction with only minimal amounts of catalyst promoter without the formation of substantial amounts of by-product formation or bisphenol-A product disintegration. Further, methyl mercaptan could be used as a catalyst promoter in a continuous process. Methyl mercaptan was also distinguished in that its high volatility allowed it to be easily separated from the reactor effluent containing bisphenol-A product and avoided the presence of sulfur contaminants in the final product. Since then, the use of gaseous free methyl mercaptan has been the catalyst promoter of choice in acid-catalyzed phenol-acetone reactions.
However, methyl mercaptan is difficult to handle because it is gaseous at room temperature and atmospheric pressure. This makes the shipping of methyl mercaptan inefficient because it must first be pressurized to a liquid state to economically transport sufficient amounts required for the manufacture of bisphenol-A to a plant located some distance from the site producing methyl mercaptan. Moreover, since methyl mercaptan is a hazardous compound, in some cases shipping this material to certain locations is restricted, resulting in limited availability in these areas. As a result, some bisphenol-A plants must now either produce methyl mercaptan on site, or switch to an alternative catalyst promoter which does not have the volatility of methyl mercaptan.
U.S. Pat. No. 6,465,697, incorporated herein by reference, discloses dithioketals, particularly bismethylthiopropane (“BMTP”), as a promoter for the acid-catalyzed condensation reaction between phenols and carbonyl compounds at substantially the same rate and with substantially the same selectivity towards bisphenol-A as methyl mercaptan and without the formation of any by-product sulfur species at levels greater than with the use of methyl mercaptan. The BMTP promoter is not as volatile as methyl mercaptan, is liquid at room temperature and atmospheric pressure and stable during transportation, thereby rendering it easily and economically transportable. BMTP catalyst promoter proved to have high activity and high selectivity. The amount of sulfur byproduct species produced using this catalyst promoter is acceptably low.